1. Field Of The Invention
The field of the invention relates to sheet type printing presses used for performing many different printing jobs without changing a printing plate.
2. Brief Description Of The Prior Art
A sheet press generally comprises one or more plate cylinders, a blanket cylinder, and an impression cylinder. One of the plate cylinders is typically driven by a power source. A gear drive is provided for coupling these two elements. Ink is provided to an etched plate mounted to the outer surface of the plate cylinder. This ink is transferred by the engagement of the outer surfaces of the plate and blanket cylinders, the latter having an outside surface defined by ink receptive blanket material. The paper to be printed passes between the blanket and impression cylinders. A precise gearing arrangement maintains the relative positions of the plate and blanket cylinders when the press is in operation.
Many printing jobs require the printing of only a few hundred or a few thousand items. Multiple images may accordingly be provided on a plate on a plate cylinder, one image for each printing job. When a job is completed, the plate cylinder is rotated with respect to the blanket cylinder until the next image is properly aligned therewith. Once the blanket cylinder has been cleaned, the next job can be run.
In order to operate efficiently, it is important for the plate cylinder to be quickly and easily rotated by the operator in a manner that will insure the selected image is properly aligned with respect to the blanket cylinder in an offset printing operation or the workpiece in direct lithography. The speed at which such a changeover can be effected is important in maximizing the number of jobs which can be performed in a selected amount of time. It is equally important for the image to be properly aligned so that workpieces are not wasted.
One approach for rotating a plate cylinder with respect to a blanket cylinder is disclosed in commonly assigned U.S. Pat. No. 5,042,380. This approach involves the use of a double harmonic drive assembly which can change the phase of a plate cylinder with respect to a blanket cylinder as the press operates.